The present invention relates generally to the field of communication systems, and more particularly to a system and method operable to facilitate controlling the shape of a noise figure generated in an optical amplifier.
Optical amplifiers generate noise through a variety of phenomena, such as when signals being amplified interact with one another and when signals being amplified interact with pump signals associated with the amplifier. Different levels of noise can be created at different wavelengths along the spectrum of wavelengths being amplified. This leads to a spectrum of noise created across the wavelengths of the amplified signals.
Although optimization techniques can be developed to counter the effects of noise generated by a particular source, the effectiveness of these techniques can deteriorate where the shape of the noise figure changes over time. Existing optimization techniques are generally not equipped to respond to phenomena that tend to change the shape of the noise figure of the amplifier.
The present invention recognizes a need for a method and apparatus operable to facilitate control of a noise figure generated in an optical amplifier. In accordance with the present invention, a system and method for controlling a noise figure reduces or eliminates at least some of the shortcomings associated with previous communication systems.
In one aspect of the invention, an optical amplifier operable to amplify a plurality of optical wavelength signals at least in part through Raman amplification comprises an input operable to receive a plurality of wavelength signals and an output operable to communicate an amplified version of at least some of the plurality of wavelength signals. The amplifier further comprises a pump assembly operable to generate one or more pump signals and a gain medium operable to receive the plurality of wavelength signals and the one or more pump signals and to facilitate amplification of at least some of the plurality of wavelength signals. The amplifier has associated with it a noise figure having a shape varying as a function of wavelength. At least one of the one or more pump signals is operable to have its power varied to selectively control the shape of the noise figure.
In another aspect of the invention, a multi-stage amplifier comprises a first amplifier stage comprising a Raman amplification stage operable to amplify a plurality of wavelength signals through interaction with one or more pump signals and a second amplifier stage operable to further amplify at least some of the plurality of wavelength signals. The power of at least one of the one or more pump signals in the first stage is operable to be varied in response to a change in power of the plurality of wavelength signals, the variation in pump power selectively controlling the shape of a noise figure of the amplifier during operation of the amplifier.
In yet another aspect of the invention, an optical amplifier operable to amplify a plurality of optical wavelength signals at least in part through Raman amplification comprises an input operable to receive a plurality of wavelength signals and a pump assembly operable to generate one or more pump signals operable to interact with one or more of the wavelength signals over a gain medium to cause Raman amplification of the one or more wavelength signals. The amplifier also comprises control circuitry operable to generate a control signal based at least in part on a signal proportional to the total power of the plurality of wavelength signals. The amplifier is operable to vary the power of at least one of the one or more pump signals in response to the control signal, the variation of the power of the at least one pump signal selectively controlling the shape of a noise figure associated with wavelength signals being amplified.
In still another aspect of the invention, a method of amplifying a plurality of wavelength signals comprises amplifying a plurality of wavelength signals and adding wavelength signals to or dropping wavelength signals from the plurality of wavelength signals. The method further comprises selectively controlling the shape of the noise figure as wavelength signals are added or dropped from the plurality of wavelength signals.
In another aspect of the invention, a method of amplifying optical signals comprises introducing to a gain medium one or more pump signals and a multiple wavelength signal comprising a plurality of wavelength signals and detecting a change in power of the multiple wavelength signal. The method also comprises adjusting a power of at least one of the one or more pump signals in response to the change in power of the multiple wavelength signal to result in selectively controlling the shape of a noise figure associated with the multiple wavelength signal.
In another aspect of the invention, an optical communication system operable to facilitate communication of multiple signal wavelengths comprises one or more transmitters operable to generate alone or collectively a plurality of signal wavelengths and a multiplexer operable to combine the plurality of signal wavelengths into a single multiple wavelength signal for transmission over a transmission medium. The system further comprises a plurality of optical amplifiers operable to receive the plurality of signal wavelengths. At least one of the optical amplifiers comprises a gain medium operable to amplify the multiple wavelength signal through interaction with one or more pump signals, the amplification occurring prior to, during, or after the multiple wavelength signal""s transmission over the transmission medium. The power of at least one of the one or more pump signals is operable to be varied in response to a change in power of the plurality of wavelength signals, the variation in pump power selectively controlling the shape of a noise figure of the amplifier during operation of the amplifier.
Depending on the specific features implemented, particular embodiments of the present invention may exhibit some, none, or all of the following technical advantages. For example, various embodiments of the invention facilitate enhanced amplifier operation by controlling the shape of a noise figure associated with all or a portion of a spectrum of amplified signals.
One aspect recognizes that it would be desirable to maintain the shape of the noise figure in an optical communication system despite changes to the system, such as variations in signal power due to, for example, wavelength signals being added to or dropped from a multiple wavelength signal. This would allow, for example, existing optimization algorithms to continue to be utilized. In addition, in some cases, the peak increase in the noise figure can be lessened by approximately maintaining the shape of the noise figure when system conditions change. Moreover, selectively controlling the shape of the noise figure can reduce or eliminate the need to monitor and adjust individual wavelength signal powers when other wavelength signal powers change.
In at least some embodiments, the shape of a noise figure of an optical amplifier can be effectively modified or maintained by altering the powers of one or more pump wavelengths, in particular pump signals at longer wavelengths. In particular embodiments, all or a majority of the shaping of the noise figure can be accomplished in a first stage of a multiple stage amplifier.
Another aspect recognizes that control signals operable to affect the shape of the noise figure can be generated based at least in part on the total power of the signals being amplified. Although the invention could equally apply to approaches using more complex spectrum analyzing techniques to ascertain a control signal, using total power to determine a control signal provides a simple and cost effective mechanism for controlling noise figure shape. In addition, the relationship between total signal power and adjustments in amplifier pump power to control noise figure shape allows for use of look-up tables or simple algorithms to determine a control signal.
Other technical advantages are readily apparent to one of skill in the art from the attached figures, description, and claims.